This invention relates to a measuring caliper instrument and more particularly, to such instruments used for measuring inside diameters and the like, such as the internal diameter of cylindrical brake drums.
Periodically it is necessary to check the wear of vehicle brake drums to determine if the drums are within a prescribed tolerance range. This tolerance range, set by the manufacturer, insures that the brake drums are still within a certain internal diameter and secondly, the degree of roundness. If a brake drum's internal diameter does not fall within the prescribed ranges, then the drum may be machined or replaced.
U.S. Pat. No. 4,782,595 shows a device used to measure the internal diameters of brake drums comprising a two-part bracket with a portion of one part slidingly received within the other part and with a dial gauge having a rotating member adapted to rotate as the parts are moved relative to one another. Pins are mounted at opposite ends of the device and are placed within a brake drum with the two-part bracket spread apart until both the pins contact the drum wall with the gauge indicating the distance between the pointers through a rack and pinion arrangement.
The device is calibrated by inserting the pins between two end tabs of a calibration bar so that a pointer of a course scale on the bracket and the dial gauge pointer indicate the known length of the calibration bar.
In order to take a measurement with the device one must add the two numbers thereby increasing the time needed to take a reading than would be required with a single scale as well as increasing the possibility of making an incorrect reading.
Linear measuring devices useful for both outside diameters as well as inside diameters are known in the art having digital display units which facilitate quick and accurate taking of measurements as shown, for example in U.S. Pat. Nos. 4,229,883 and 4,972,603. Both of these devices incorporate an elongated beam member on which is disposed circuit means which cooperate with a caliper member mounting a battery powered digital display unit which has pick-up and counting means responsive to the circuit means on the beam so that the amount of movement on the caliper member is successively digitally displayed. The digital display units are provided with re-setting means so that the display counter can be calibrated by being made zero at a selected position, for example, when inside dimension measuring bill 3 of Pat. No. 4,229,883 passes moveable bill 17 when their measuring surfaces come into alignment or as shown in FIG. 7 of Pat. No. 4,972,603 when measuring jaws 88, 81 pass one another so that their measuring surfaces come into alignment.
However the provision of measuring arms which are configured and mounted in such a manner as to pass one another and still provide accurate, reliable movement, is expensive and relatively complex.
It is an object of the invention to provide an improved linear measuring instrument having digital display means particularly suitable for measuring inside dimensions such as internal diameters.
Another object is the provision of such an instrument which is relatively inexpensive and yet is reliable and is easily used in a consistent manner from one user to another.
Yet another object is the provision of a method for calibrating to a selected standard dimension a measuring instrument particularly suited for measuring inside diameters.
Briefly, in accordance with the invention, calipers adapted for measuring internal diameters and the like having a digital display unit are provided with first and second jaw members slideably mounted on an elongated beam. Each jaw member has an arm mounting a measuring tip extending in a first direction with the arms both lying in the same plane. The first jaw member comprises a frame formed with a beam receiving recess and a groove in communication with the recess. A stop surface is disposed in the groove which interacts with a stop pin mounted on the beam adjacent an end thereof and moveably received within the groove. Set screws are mounted in the frame to selectively fix the frame to the beam and an adjustment screw moveable in the direction of the longitudinal axis of the beam extends from the side of the frame closest to the second jaw member toward the second jaw means. The second jaw member comprises a housing mounting the digital display unit which is provided with a needle receiving resetting aperture.
According to a feature of the invention, one side of the frame is placed flush with one side of the beam and the frame is then fixed to the beam by the set screws. With the two jaw members in engagement with one another, the adjustment screw is rotated until the distance between the measuring tips equals a selected calibration distance at which point the adjustment screw is fixed in place. The caliper instrument is then in condition to provide to an end user.
According to a feature of the invention, an end user calibrates the display unit by loosening the set screws and, with the two jaw members held in engagement with one another, moving the beam to the right as seen in FIG. 4 until the stop pin engages the stop surface. A calibration needle is then inserted into the resetting aperture of the display unit to cause the display to show zeros. The beam is then moved in the opposite direction, i.e. to the left as seen in FIG. 4, until the selected calibration distance is displayed and the set screws are retightened to fix the position of the frame to the beam. The instrument will then remain in its calibrated condition until battery replacement or in the event that the original factory calibration is changed.